Two aircraft can come dangerously close either because of a lapse by a pilot or air traffic control, or because of a communication problem. What contributed to the near-collision today will only be clear after a probe.

Both pilots received alerts through the traffic collision avoidance system (TCAS), a software installed in aircraft that warns the pilot when another plane comes too close.

IndiGo’s Flight 6E 472 had taken off from Bagdogra at 12.22pm and was cleared by air traffic control to climb to 30,000 feet, an official said.

“The Air India flight was descending. The captain (of the IndiGo flight) got a TCAS resolution advisory. As per the standard operation procedure, the captain followed the advisory and descended,” an IndiGo spokesperson said.

“The Air India flight also followed the RA (resolution advisory) and turned right. Once the captain received a ‘clear of conflict’ message, they (the pilots) manoeuvred back to normal situation.”

She said the IndiGo flight landed in Delhi at 2.15pm.

The traffic collision avoidance system issues two types of safety advisories — traffic and resolution.

“A traffic advisory is issued when the flights are close to each other but not close enough to be on a collision course. A resolution advisory is issued when they are within 39 seconds of flying distance from each other,” a senior pilot with a private airline said.

In the system, a voice asks the pilot to either descend or ascend to avoid collision. At the same time, the altitude and position of the flight is displayed on a screen in front of the pilot.

Air India officials said they had no information about the incident. Bagdogra’s air traffic control is controlled by the Indian Air Force. An Air Force spokesperson said he had no information about the incident.

Sources said the incident would be probed by the directorate-general of civil aviation since two civilian aircraft were involved.

According to sources in the directorate — the civil aviation regulator — there is normally a separation of five nautical miles or 9.26km between two flights if they are at the same level and there is radar coverage.

At places where there is no radar coverage, the separation is 10 nautical miles or 18.52km. “The vertical separation between two flights is usually 1,000 feet,” a directorate-general of civil aviation official said.